The present invention relates to ultrasonic sensors for measuring physical properties of materials. More particularly, the present invention relates to a controller for an ultrasonic sensor used in a 4-20 mA communication loop.
Various ultrasonic distance and level measuring and level measuring systems are known. For example, Ellinger et al. U.S. Pat. No. 4,815,323 teaches an "echo ranging" ultrasonic transducer transmitting an ultrasonic signal which is reflected from a liquid surface in an aircraft fuel container and then sensed. The round-trip time from sensing to receiving is measured and the fuel quantity and density are computed in a central processing unit as a function of the round trip time and stored data.
In another type of ultrasonic sensor as taught by U.S. Pat. No. 4,299,114 to Silvermetz et al., an ultrasonic transmission between transmit and receive transducers closes a feedback loop of a circuit which oscillates when feedback increases to a predetermined amount. When a material level in a container rises to a level where the transmit and receive transducers are mounted, the increased feedback through the material causes the circuit to oscillate. This type of sensor requires a relatively large difference between the amount of feedback when sensing a liquid and the amount of feedback when sensing a gas for stable operation. Undesired feedback through a sensor body can reduce stability, as well. If air is entrapped in the material, feedback can be reduced and sensing fails. In this type of sensor, misalignment of the transducers can reduce feedback through the materials so that sensing fails.
Ultrasonic transducers require a controller to control their operation. The controller operates the transducer elements, performs sensor test functions and provides a sensor output signal. There is a continual need for improvements in controllers used with ultrasonic sensors.